Returns a query string for this up.Params instance.
The keys and values in the returned query string will be percent-encoded.
Non-primitive values (like File) will be omitted from
the returned query string.
let params = new up.Params({ foo: 'bar', baz: 'bam' })
let query = params.toQuery()
// query is now: 'foo=bar&baz=bam'
A query string built from this up.Params instance.
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